1. Field of the Invention
The present invention relates to a method for the preparation of metallic oxides from alkoxides of the corresponding metals.
2. Discussion of the Background
Methods for preparation of the polymer gels of metallic oxides by hydrolyzing alkoxides of the corresponding metals are known. In particular, very fine powders of metallic oxides have been prepared from these gels, using pyrolysis.
Thin layers of metallic oxides, as well as their use in conductive or antistatic layers, microwave oven doors, transparent electrodes (for solar cells), heated windowpanes as well as screens used in electronics, are also known.
However, it is difficult to form layers, particularly from alkoxides whose hydrolysis is very rapid. This rapid hydrolysis leads to rapid setting and thus formation of a solid gel which can no longer be layered. In order to prepare layers, the support is usually first soaked in the alkoxide solution, then the solution, already applied, is hydrolyzed. Thus, Cd stannate or Ti or Sb stannate layers obtained by soaking a support in an alcoholic solution of metallic oxides are described in patents GB 2 009 723 and US 3 647 531 and in "History and principles of the sol-gel process and some new Multicomponent oxide coatings", H. Dislich and P. Hinz, Journal of Non Crystalline Solids, 48 (1982) 11-16. The layer is hydrolyzed after its application by drying a humid atmosphere, then heating at around 500.degree.-650.degree. C. These products are used as electroconducting materials or to make windows, microwave oven doors, solar panels, etc.
Nonetheless, such a method does not allow the viscosity of the solution to be controlled. It is known that polymer gels are the precursors of the final oxide desired and that control of the formation of these polymers would thus allow the structure of the final oxide obtained to be controlled. Obtaining a homogeneous polymer structure is thus essential.
One method has been described in the prior art, especially in "Sol-Gel Processing of Silica I. The role of the starting compounds", K. C. Chen, T. Tsuchiya and J. D. Mackensie, Journal of Non Crystalline Solids, 81 (1986) 227-237, and in "Study of Polymerization Processes in Acid and Base Catalyzed Silica Sol-Gel", T. W. Zerda, J. Artaki and J. Jonas, Journal of Non Crystalline Solids, 81 (1986) 365-379. It consists in adding a water/alcohol mixture dropwise to an alkoxide solution in alcohol. Nonetheless, this method does not give very homogeneous gels. In fact, during addition of water, the concentration of water in the solution is not homogeneous at all. It is very high around the drop added and very low everywhere else. Although this method of direct hydrolysis is suitable for silicon alkoxides, it is not suitable for alkoxides of other metals which are very unstable and which hydrolyze very rapidly, even simply due to humidity in the air. A method for hydrolysis of a metal alkoxide dissolved or dispersed in a water in oil emulsion has also been suggested (Chem. Abst. vol. 106, no 2, January 1987, p. 118, No 7090v). In this method, in which no surfactant is used, an emulsion of waterdrops in oil forms and hydrolysis of the alkoxide takes place in the waterdrops. The oxide is consequently formed in the water, from which it is extracted by freezing to be obtained in solid form. This method has some disadvantages: the rate of hydrolysis cannot be controlled and a metallic oxide layer on a support cannot be directly obtained.
Thus, it is most desirable to find a method for hydrolysis of metal alkoxides allowing viscous polymer solutions having a preformed homogeneous structure and a viscosity adapted to layering on a support to be obtained and allowing the thickness of the layer to be controlled.